I. Field of the Invention
The present invention relates to the field of glazing that can be used alternatively or cumulatively in three particular applications, namely thermal control (solar control and thermal insulation), electromagnetic shielding and heated windows, while still preferably being able to undergo at least one transformation operation involving a heat treatment at a temperature of at least 500° C. (this may be in particular a toughening, annealing or bending operation).
Thermal control is the possibility of acting on solar radiation and/or long-wavelength infrared radiation passing through glazing that separates an outdoor environment from an indoor environment, either for outward reflection of the solar radiation (“solar control” glazing) or for inward reflection of the infrared radiation of wavelength greater than 5 μm (thermal insulation with glazing called in particular “low-emissivity glazing”).
Electromagnetic shielding is the possibility of eliminating, or at the very least reducing, the propagation of electromagnetic waves through glazing. This possibility is often combined with the possibility of acting on the infrared radiation passing through the glazing. This application is advantageous in the electronics field, especially for the production of electromagnetically shielded windows, also called “electromagnetic filters”, which are intended for example to be placed on the faceplate of a display screen using plasma technology.
A heated window is a window whose temperature may rise when it is subjected to an electrical current. This type of window has applications in automobiles, or even in buildings, for the production of glass panes that prevent the formation, or that eliminate, ice or misting, or else that suppress the sensation of a cold wall near the glazing.
The present invention relates more particularly to a transparent substrate, especially made of glass, which is provided with a thin-film stack comprising a plurality of functional layers, said substrate being able to be used, alternatively, or cumulatively, for thermal control, electromagnetic shielding and heated windows.
II. Description of Related Art
It is known to produce thin-film stacks to achieve thermal control, more precisely solar control, which are capable of simultaneously preserving their thermal properties and their optical properties after heat treatment, while minimizing any appearance of optical defects, the challenge then being to therefore have thin-film stacks of constant optical/thermal performance, whether or not they subsequently undergo one or more heat treatments.
A first solution was proposed in European patent application No EP 718 250. This recommended the use, above the silver-based functional layer(s), of oxygen-diffusion barrier layers, especially those based on silicon nitride, and the direct deposition of silver layers on the subjacent dielectric coating, without interposition of priming layers or metal protection layers. That patent application describes in particular a stack of the type:                substrate/Si3N4 or AlN/ZnO/Ag/Nb/ZnO/Si3N4.        
A second solution was proposed in European patent application No EP 847 965. This relies on stacks comprising two silver layers and describes the use both of a barrier layer on top of the silver layers (as previously) and of an absorbent or stabilizing layer which is adjacent to said silver layers and allows them to be stabilized.
That patent application describes in particular a stack of the type:                substrate/SnO2/ZnO/Ag1/Nb/Si3N4/ZnO/Ag2/Nb/SnO2/Si3N4.        
In the above two solutions, it should be noted that the presence of the absorbent “overblocker” metal layer, made of niobium in this case, or even titanium, on the silver layers makes it possible to prevent the silver layers from coming into contact with an oxidizing or nitriding reactive atmosphere during deposition by reactive sputtering of the SnO2 layer or the Si3N4 layer respectively.
A third solution has since been disclosed in International patent application No WO 03/01105. This proposes to deposit the absorbent “blocker” metal layer not on the (or each) functional layer, but underneath, so as to allow the functional layer to be stabilized during the heat treatment and to improve the optical quality of the stack after heat treatment.
That patent application describes in particular a stack of the type:                substrate/Si3N4/ZnO/Ti/Ag1/ZnO/Si3N4/ZnO/Ti/Ag2/ZnO/Si3N4.        
However, within the thickness ranges disclosed, such a stack cannot be used to produce a heated window or electromagnetically shielded window of acceptable appearance (optical characteristics).
The prior art also teaches thin-film stacks on a substrate that can be used for thermal control and for heated windows when they are subjected to an electrical current. International patent application No WO 01/14136 thus discloses a stack consisting of a silver double layer that withstands a toughening heat treatment, which can be used for solar control and for producing heat when it is subjected to an electrical current. However, the resistivity of this stack does not allow true effective electromagnetic shielding to be achieved as its resistance per square R□ cannot be close to, and a fortiori less than, 1.5 ohms per square.
In addition, for heated window application in automobiles, this high resistance per square requires the use of a battery with a high voltage across its terminals (around 42 volts, the standard voltage available on the market) in order to be able to heat over the entire height of the window. Specifically, by applying the formula P(W)=U2/(R□×h2), if R□=1.5 ohms per square, to achieve P=600 W/m2 (the estimated power dissipated for correct heating) and to obtain a heating height h>0.8 meters, it is necessary that U>24 volts.
It is also known to produce thin-film stacks for electromagnetic shielding using a substrate provided with an electromagnetic protection stack providing good electromagnetic protection and allowing the user to easily see the image display thanks to a high light transmittance together with a low reflectance.
To achieve electromagnetic shielding, the prior art also teaches, from International patent application No WO 01/81262, a stack in particular of the type:                substrate/Si3N4/ZnO/Ag1/Ti/Si3N4/ZnO/Ag2/Ti/ZnO/Si3N4.        
This stack can withstand a toughening or bending heat treatment. However, this stack does not make it possible to achieve a resistance per square that is much less than 1.8 ohms per square with optical characteristics (TL, RL, color, etc.) that are deemed to be acceptable, and especially a low light reflection RL in the visible.
Silver-film-based stacks are manufactured in very complex manufacturing units.
The major drawback of the prior art lies in the fact that it is essential to make major modifications to the production line when it is desired to use the production line to manufacture, on the substrate, a thin-film stack that does not have the same application(s) as the stack manufactured previously on this same line.
In general, this operation lasts from several hours to several days, it is tedious and entails a very substantial loss of money, as it is not possible to produce glazing during this transition/adjustment period.
In particular, whenever the material of the target differs from one product to the next, the chamber must be returned to atmospheric pressure before the target is changed, then the chamber must be pumped down to a vacuum (of the order of 10−6 bar), which is obviously time consuming and tedious.